Mitosis is the process of nuclear division in eukaryotic cells that produces two daughter cells from one parent cell. The daughter cells and the original parent cell have identical chromosomes and DNA. Generally, cancer is a disease of mitosis. It is believed that cancer begins when a single cell is converted from a normal cell to a cancer cell. This is often due to a change in function of one or more genes that normally function to control cell growth. The cancer cells proliferate by repeated, and uncontrolled mitosis, in contrast to normal cells which undergo only about 20 to 50 generations of replication and then cease. A tumor may be thought of a mass of unhealthy cells that are dividing and growing in an uncontrolled way.
Microtubules are long, protein polymers that are hollow, tube-like filaments found in certain cell components such as the mitotic spindle. Each microtubule is composed of repeating subunits of the protein tubulin. Microtubules aggregate to form spindle fibers. During mitosis, cells use their spindle fibers to line up chromosomes, make copies of them, and divide into new cells with each new daughter cells having a single set of chromosomes. The polymerization dynamics of microtubules play a pivotal role in this process as part of cell replication. The crucial involvement of microtubules in mitosis makes them a target for antitumor agents. Antitumor agents that inhibit the function of microtubules are known as antimitotic agents.
Many classes of antimitotic agents are known. One such class is the vinca alkaloids exemplified by vincristine, vinblastine, vindesine, and vinorelbine. The vinca alkaloids are used in the treatment of leukemias, lymphomas, and small cell lung cancer. Another class of antimitotic agents are the taxanes, exemplified by paclitaxel (commercially available from Bristol-Myers Squibb Company under the tradename TAXOL®) and docetaxel. The taxanes are useful in the treatment of breast, lung, ovarian, head and neck, and bladder carcinomas. Colchicine typifies another class of antimitotic agents. Colchicine, while not used as an antitumor agent, is a microtubule polymerization inhibitor. Lastly, the combrestatins are another class of antitumor agents. Antimitotic agents such as the vinca alakaloids, colchicine, colcemid, and nocadazol block mitosis by keeping the mitotic spindle from being formed. These agents bind to the tubulin and inhibit polymerization, preventing cells from making the spindles they need to move chromosomes around as they divide. In contrast, paclitaxel binds to the tubulin protein of microtubules, locking the microtubules in place and inhibiting their depolymerization. With the mitotic spindle still in place, a cell may not divide into daughter cells.
Multidrug or multiple drug resistance (MDR) is a major drawback of cancer chemotherapy. Ultimate failure of chemotherapy often times occurs with the use of antimitotic agents due to MDR. MDR may be inherently expressed by some tumor types while others acquire MDR after exposure to chemotherapy. P-glycoprotein (Pgp) is a 170 kilodalton (kDa) protein that belongs to the ATP-binding cassette superfamily of transporters. Pgp has been implicated as a primary cause of MDR in tumors. Pgps are efflux transporters found in the gut, gonads, kidneys, biliary system, brain, and other organs. A series of homologous proteins termed multidrug-resistance proteins (MRPs) are also known. MRPs are associated with MDR in tumors. The first MRP termed MRP1 was identified in a drug resistant lung cancer cell line that expressed Pgp. All of these transporters bind drugs within cells and release them to the extracellular space using ATP. Tumor cells pre-exposed to cytotoxic compounds often allow the cells to manifest resistance in the presence of the cytotoxic drug. Overexpression of Pgp has been reported in a number of tumor types, particularly after the patient has received chemotherapy, indicating the clinical importance of Pgp in MDR. The clinical significance of Pgp along with its limited expression in normal tissues makes Pgp a viable target for inhibition to reverse MDR.
While antimitotic agents have shown to be some of the most successful agents against malignancies, resistance, both intrinsic and acquired, often results in treatment failures. Thus, there exists a need to develop new compounds that possess antimitotic activity, anti-multidrug resistance activity, and antitumor activity, that may be used alone as a single agent in the treatment of cancer, or in combination with chemotherapeutic agents, including antimitotic agents, that shall inhibit mitosis in a wide variety of cells, including cells that are subject to MDR. There is a need, therefore, for single compounds which provide the desired antimitotic, anti-multidrug resistance and antitumor activities with a high degree of selectivity and low toxicity, and that are effective inhibitors of paclitaxel sensitive and resistant tumor cells.